1. Field of the Invention
The present invention relates generally to a control system for an internal combustion engine equipped with an automatic transmission for driving a motor vehicle such as an automobile, car and so forth. More particularly, the present invention is concerned with an automatic-transmission-equipped engine control system which is capable of controlling a driving power at a minimum fuel consumption even in the nonlock-up state of input/output elements of an input clutch (i.e., even when the input clutch which may be constituted by a torque converter including a fluid coupler portion are not in the state where the input element and the output element thereof are directly or straight-forwardly coupled with each other.
2. Description of Related Art
In general, the automatic transmission operatively coupled to an internal combustion engine (hereinafter also referred to simply as the engine) includes an input clutch and a continuously variable transmission (also termed CVT in short) for regulating variably the transmission gear ratio in a stepless manner or continuously.
In the automatic-transmission-equipped engine control system of this type, it is required to optimize the engine operation range with a view to enhancing the fuel-cost performance by resorting to cooperative control of the engine and the continuously variable transmission (CVT) regardless of whether the input clutch is in the nonlock-up state or in a changeover or switching phase intervening between the nonlock-up state and the lock-up state.
As the conventional automatic-transmission-equipped engine control systems known heretofore, there may be mentioned driving power control systems for the motor vehicles such as those described below.
In the first place, reference may be made to Japanese Patent Publication No. 62263/1993. In this publication, there is disclosed such an arrangement of the conventional automatic-transmission-equipped engine control system in which the desired or target driving power demanded by the driver is corrected or modified in dependence on the operating state of the engine, whereon target values of the control parameters for the engine and the automatic transmission are arithmetically determined.
At this juncture, it should be mentioned that the desired or target level or value of the driving power is basically determined on the basis of the accelerator pedal actuation quantity (hereinafter also referred to as the accelerator pedal stroke) and the speed of the motor vehicle on which the engine is mounted. This speed will hereinafter be referred to as the motor vehicle speed or simply as the vehicle speed or car speed. The target value of the driving power thus determined is then corrected in view of the engine operating state such as the rate of change of the accelerator pedal stroke, the slope of a road on which the motor vehicle is running and the running state of the motor vehicle such as the weight of the motor vehicle and the like.
As the target control parameters for the target driving power, there are arithmetically determined the target engine torque and the target transmission gear ratio (target value of the ratio in the rotation speed (rpm) between the input and output shafts of the continuously variable transmission or CVT).
In succession, the engine torque is modified or corrected so that the actual driving power and the target or demanded driving power coincides with each other, while the engine output control unit is so controlled that the transmission gear ratio of the continuously variable transmission (CVT) coincides with the target transmission gear ratio (i.e., target value of the transmission gear ratio).
In this manner, not only the fuel-cost performance of the motor vehicle is improved but also the engine output power control can be realized with enhanced response performance in the transition state of the motor vehicle. Thus, the excellent maneuverability of the motor vehicle can be realized while realizing the target driving power, i.e., driving power demanded by the driver.
Further, Japanese Patent Application Laid-Open Publication No. 332446/1995 (JP-A-7-332446) discloses a speed change control unit for the continuously variable transmission (CVT) in which a speed-change map data prepared with importance being put on to the fuel-cost performance and a speed-change map data prepared with importance being put on the engine power are employed, wherein the transmission gear ratio is determined by interpolating each of the speed-change map data in accordance with a signal bearing correlation to the rate of change of the engine load.
In this way, the speed change control which ensures the most appropriate engine output performance can be carried out in conformance with the acceleration demanded by the driver.
Furthermore, in the continuously variable transmission (CVT) described in Japanese Patent Application Laid-Open Publications Nos. 1135/1999 and 324176/1998, a torque converter or the like is employed as the input clutch, wherein in the state where the input and output shafts of the torque converter are not directly intercoupled (i.e., in the nonlock-up state or in the converter state, so to say), the target transmission gear ratio and the target engine torque are determined.
By way of example, according to the teaching disclosed in Japanese Patent Application Laid-Open Publication No. 1135/1999, the target engine torque is corrected in dependence on the operating state of the mechanism for interrupting intermittently the CVT interrupting mechanism such as the start clutch, torque converter or the like.
Furthermore, according to the teaching disclosed in Japanese Patent Application Laid-Open Publication No. 1135/1999, control of the input shaft power (target driving power) of the automatic transmission (T/M) is realized in the state where the fuel consumption ratio is most preferable at the time point when the stable operation point at which the engine output torque coincides with the absorption torque of the automatic transmission has been reached.
However, with the conventional automatic-transmission-equipped engine control systems (i.e., driving power control systems) in which the input clutch typified by the fluid coupler such as the torque converter, for example, is employed as the intermittently interrupting mechanism of transmission, there arise inconveniences such described below when such a combination of the engine speed (rpm) and the engine torque is selected which can minimize the fuel consumption ratio in the nonlock-up state of the input/output elements of the input clutch.
More specifically, when such target engine speed and target engine torque which can minimize the fuel consumption in the nonlock-up state mentioned above are selected for controlling correspondingly the continuously variable transmission (CVT) and the intake air quantity regulating unit (throttle actuator), the speed ratio between the input and output shafts of the torque converter will become greater than xe2x80x9c1xe2x80x9d (one) or the engine speed (rpm) becomes greater than the input speed of the continuously variable transmission (CVT), to say in another way. Consequently, even when the continuously variable transmission (CVT) is controlled optimally, control to realize the demanded or target engine speed is impossible, incurring a problem that the target driving power as demanded by the driver can not be met.
For coping with the problem mentioned above, the engine control system described, for example, in Japanese Patent Application Laid-Open Publication No. 1135/1999 is so arranged as to modify or correct the output shaft torque of the continuously variable transmission (CVT) by dividing it by a torque ratio which is based on the speed ratio between the input shaft and the output shaft of the torque converter.
Owing to the arrangement described above, the driving power control accuracy can certainly be enhanced even in the nonlock-up state of the torque converter. Besides, in the engine control system disclosed in the above-mentioned publication, the arithmetic determination of the target transmission gear ratio and that of the target driving power are executed independently from each other. Consequently, improvement of the maneuvering performance of the motor vehicle can be made compatible to the improvement of the fuel-cost performance to a certain extent.
However, because the output shaft torque of the continuously variable transmission (CVT) is corrected by dividing it by the torque ratio of the torque converter, the driver""s driving power demand can not always be met at the minimum fuel consumption through the control based on the combination of the corrected target transmission gear ratio (engine speed (rpm)) and the target engine torque.
On the other hand, in the engine control system disclosed in Japanese Patent Application Laid-Open Publication No. 324176/1998, the target driving power is controlled in the minimum fuel consumption state when the stable operation point has been attained. However, the conditions for allowing the stable operation point to be reached are limited. In this conjunction, it is noted that the control state of minimum fuel consumption can not be realized at any other point than the stable operation point.
As is obvious from the foregoing, the conventional automatic-transmission-equipped engine control systems known heretofore suffer a problem that the driving power control for realizing the minimum fuel consumption can not be realized in the case where the input and output elements of the input clutch such as the torque converter or the like are not in the lock-up state.
In the light of the state of the art described above, it is an object of the present invention to provide a control system for an internal combustion engine equipped with an automatic transmission, which control system can realize the fuel-cost performance improvement with the operation range being optimized by correcting the target driving power (i.e., target value of the driving power) in conformance with the transmission efficiency when the transmission efficiency of the input clutch is low (i.e., when it is smaller than 100%), while making the control possible at the operation point of the minimum fuel consumption (maximum fuel-cost efficiency) even in the nonlock-up state.
Another object of the present invention is to provide a control system for an internal combustion engine equipped with an automatic transmission, which control system is capable of performing the control at the minimum fuel consumption even in the nonlock-up state by arithmetically determining a target transmission input speed in dependence on the speed ratio between the input and output elements of the input clutch.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a control system for an internal combustion engine equipped with an automatic transmission, which system includes a set of various types of sensors for detecting operating state of the internal combustion engine, an intake air quantity regulating unit for regulating adjustably an intake air quantity of the internal combustion engine, an automatic transmission including an input clutch and a continuously variable transmission (CVT) coupled operatively to the engine for regulating a transmission gear ratio continuously variably, and a control unit for controlling the engine and the automatic transmission on the basis of the operating state of the engine, wherein the control unit is comprised of a target driving power arithmetic module for arithmetically determining a target driving power of the engine on the basis of the operating state of the engine, a transmission efficiency arithmetic module for arithmetically determining a transmission efficiency of the input clutch, a target driving power correcting module for correcting the target driving power in dependence on the transmission efficiency to thereby determine arithmetically a corrected target driving power, a target value selecting module for selecting a combination of a target engine speed and a target engine torque for realizing the corrected target driving power, a target intake air quantity arithmetic module for arithmetically determining a target intake air quantity for realizing the target engine torque, and a target transmission input speed arithmetic module for arithmetically determining a target transmission input speed of the continuously variable transmission so that the input speed of the automatic transmission coincides with the target engine speed, wherein the intake air quantity regulating unit is so controlled that the intake air quantity of the engine coincides with the target intake air quantity, and wherein the transmission gear ratio is so controlled that the input speed of the continuously variable transmission coincides with the target transmission input speed.
By virtue of the arrangement of the control system for the internal combustion engine equipped with the automatic transmission described above, the fuel-cost performance improving control with the optimalized operation range can be realized. Besides, even in the nonlock-up state, the aimed or demanded control can be carried out at the operation point corresponding to the minimum fuel consumption (or at point corresponding to the maximum fuel consumption efficiency, to say in another way).
In a preferred mode for carrying out the invention, the target driving power correcting module may be constituted by a division module designed for dividing the target driving power by the transmission efficiency.
With the arrangement mentioned above, the control can be performed at the operation point corresponding to the minimum fuel consumption even in the nonlock-up state of the transmission.
In another preferred mode for carrying out the invention, the transmission efficiency arithmetic module may be so designed as to correct the transmission efficiency in dependence on the engine operating state.
Owing to the arrangement described above, the control can be performed at the operation point which can ensure the minimum fuel consumption independent of the engine operating state because the transmission efficiency is corrected in dependence on the engine operating state.
In yet another preferred mode for carrying out the invention, the transmission efficiency arithmetic module may include a lower limit value setting means for setting a lower limit value of the transmission efficiency.
With the arrangement described above, the target driving power can positively be protected against abnormal correction due to the transmission efficiency smaller than the lower limit value.
In still another preferred mode for carrying out the invention, the transmission efficiency arithmetic module may be so designed as to set the transmission efficiency to a given constant value when the transmission efficiency is not greater than a predetermined value.
By virtue of the arrangement described above, the control system which does not impair the driver""s feeling of comfortableness can be realized.
In a further preferred mode for carrying out the invention, the transmission efficiency arithmetic module may be so designed as to set the transmission efficiency to a given constant value when operating state of the engine indicates that the vehicle speed is decelerating.
Owing to the arrangement described above, the control system which does not impair the driver""s feeling of comfortableness can be realized.
In yet further preferred mode for carrying out the invention, the transmission efficiency arithmetic module may be so designed as to set the transmission efficiency to a given constant value when a speed ratio between an input element and an output element of the input clutch is decreasing.
With the arrangement described above, the engine control system which does not impair the driver""s feeling of comfortableness can be realized.
In still further preferred mode for carrying out the invention, the input clutch may be comprised of a fluid type torque converter which includes a lock-up mechanism. The control unit may include a lock-up state detecting module for detecting a complete lock-up state of the lock-up mechanism. Correction of the target driving power can then be inhibited so long as the complete lock-up state is detected by the lock-up state detecting module.
With the arrangement of the automatic-transmission-equipped engine control system described above, the control system which does not impair the driver""s feeling of comfortableness due to unnecessary correction of the target driving power can be realized.
In a preferred mode for carrying out the invention, the transmission efficiency arithmetic module may be so designed as to set the transmission efficiency fixedly to 100% when the lock-up mechanism is in the complete lock-up state, to t hereby inhibit the correction of the target driving power.
With the arrangement described above, the control system which does not impair the driver""s feeling of comfortableness due to unnecessary correction of the target driving power can be realized.
In another preferred mode for carrying out the invention, the lock-up state detecting unit may be so designed as to detect the complete lock-up state of the lock-up mechanism, when difference in speed between an input shaft and an output shaft of the torque converter is less than a predetermined speed inclusive.
With the structure mentioned above, the complete lock-up state can be detected without fail.
In yet another preferred mode for carrying out the invention, the input clutch may be comprised of a fluid type torque converter, and the transmission efficiency arithmetic module may be so designed as to correct the transmission efficiency in dependence on a temperature of a fluid accommodated within the automatic transmission.
Owing to the arrangement mentioned above, the target driving power can accurately be corrected regardless of the temperature prevailing within the automatic transmission.
In still another preferred mode for carrying out the invention, the control unit may include a speed ratio arithmetic module for arithmetically determining a speed ratio on the basis of a ratio between an input speed and an output speed of the input clutch, and the transmission efficiency arithmetic module may determine arithmetically the transmission efficiency on the basis of the speed ratio.
With the arrangement of the automatic-transmission-equipped engine control system described above, there can be realized the control system which does not impair the driver""s feeling of comfortableness and which can ensure the control of minimum fuel consumption.
In a further preferred mode for carrying out the invention, the transmission efficiency arithmetic module may be so designed as to impose limitation onto the transmission efficiency on the basis of the speed ratio.
With the arrangement described above, there can also be realized the control system which does not impair the driver""s feeling of comfortableness.
In yet further preferred mode for carrying out the invention, the control unit may include a speed ratio arithmetic module for arithmetically determining a speed ratio on the basis of a ratio between an input speed and an output speed of the input clutch, and the target transmission input speed arithmetic module may be so designed as to multiply the target engine speed by the speed ratio to thereby determine arithmetically the target transmission input speed.
Owing to the arrangement of the automatic-transmission-equipped engine control system described above, there can be realized the control system which does not impair the driver""s feeling of comfortableness and which can ensure the control at the minimum fuel consumption even in the nonlock-up state.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.